The present invention relates to a road speed detection device and method for a vehicle adapted for four wheel drive operation and incorporating a four wheel drive power transmission system, and more particularly relates to such a detection device and method for the road speed of a vehicle such as an automobile incoprorating such a four wheel drive power transmission system, said road speed generally being required to be detected in order to provide proper operation of an automatic transmission mechanism of said vehicle, said road speed detection device and method not being adversely affected by slipping or skidding either of the front wheels of the vehicle or of the rear wheels of the vehicle.
The present invention has been described in Japanese Patent Applications Ser. Nos. Showa 61-066527 and Showa 61-102804 (1986), both filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent appication; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is riding is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the so called full time four wheel drive type of transmission, which remains always engaged to four wheel drive without any episodes of two wheel driving, is becoming more and more popular.
In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front - rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of the so called tight corner braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof (and also for various other reasons). And such provision of such a central or front - rear differential device is effective for achieving this result.
Further, it has been practiced to provide an automatic transmission system to a vehicle which is equipped with such a four wheel drive type transmission. Such a type of structure is disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 56-138020 (1981). In such an automatic transmission system for a vehicle which is equipped with such a four wheel drive type transmission, as is per se conventional, the speed stage of the transmission mechanism is selected and is set by a transmission control device, based upon preset patterns stored in a memory thereof, according to the current values of various vehicle operating parameters including a parameter representing engine load (such as the opening of a throttle valve of the engine of the vehicle) and a parameter representing vehicle road speed. Therefore, it is required to ascertain the value of such a parameter representative of vehicle road speed; and conventionally this has been done by detecting the rotational speed of a member which rotates at a speed proportional to the rotational speed of one or another of the vehicle road wheels, or alternatively by detecting the rotational speed of a member which rotates at a speed proportional to the average rotational speed of the rear wheels of the vehicle, or at a speed proporational to the average rotational speed of the front wheels of the vehicle. Such a member, for example, may be an output member of such a central or front - rear differential device for driving a differential device which distributes rotational power between the two rear wheels of the vehicle, or may be an output member of such a central or front - rear differential device for driving a differential device which distributes rotational power between the two front wheels of the vehicle.
However, a problem that arises with such a simplistic device and method for detecting vehicle road speed, in such a vehicle which is equipped with a four wheel drive power transmission system, is that, if at any time even one only of the four vehicle wheels should break away from the road surface and should start to spin, i.e. should slip or should skid, then the rotational speed of that wheel, and also the average rotational speed of that wheel and the other wheel on its axle (the other front wheel or the other rear wheel) will substantially increase. If, therefore, the control of the automatic transmission system for the vehicle is being done based upon the rotational speed of said one of the wheels which is slipping or skidding, or upon the average of the rotational speed of said one of the wheels which is slipping or skidding and of the rotational speed of the other wheel on its axle, then a spurious value of vehicle road speed will be utilized for such vehicle automatic transmission system control, and this is likely to result in the automatic transmission of the vehicle being set to an inappropriate speed stage for the actual current true value of the vehicle road speed. This type of problem is particularly likely to arise in the event that the road conditions are poor due to rain, snow, dust, dirt, or the like which deteriorate the coefficient of friction of the vehicle wheels on the road surface, and thereby vehicle drivability can be severely reduced.